The lentivirus, equine infectious anemia virus (EIAV), displays a highly restricted cell type preference both in vitro and in vivo. Additionally, like other members of the lentivirus family, EIAV is subject to antigenic variation as evidenced by changes that occur in envelope glycoproteins during the course of infection. To further understand the restrictive host range and correlate proviral changes that occur during pathogenesis, we have molecularly cloned and sequenced an EIAV provirus. Comparison of the gag and pol genes of EIAV with the human immunodeficiency virus and the visna virus clearly establishes that EIAV is genetically related and equally divergent from these two distinct lentiviruses. Additionally, we have performed DNA- mediated transfection analysis and viral infectivity assays of DNA isolated from a productively infected dog cell line (EIAV cf-2). Results from these experiments indicate that some proviruses harbored in this cell are biologically active. We have molecularly cloned 23 of these proviruses using lambda vectors and are currently assaying each for infectivity.